I work at an ISP of sorts. It is a regional research and education network. We are owned by our members. Heterogeneity is definitely par for the course, but that does not stop us from trying to roll out some ubiquity where we can. Many of our CPE routers are the same make and model, and that makes maintenance and analysis much less error prone. I.e. better service for our member institutions.

If you want an ISP whose competitive advantage is dealing with whatever crazy shit the edge throws at it, then that is your prerogative. But having some ground rules and baseline behavior makes it so that the ISP can focus on more rewarding tasks, such as negotiating peerings, establishing direct tunnels, improving network observability, and predicting necessary backbone upgrades.